Mucin 1 (MUC1) is a transmembrane protein of the mucin family, which is heavily glycosylated in its extracellular domain and is characterized by polarized expression on apical epithelial cell surfaces. In diverse cancer types, e.g., epithelial adenocarcinomas, MUC1 is overexpressed, the expression pattern loses its polarity, and it is aberrantly underglycosylated. Therefore, the abnormal MUC1 form has been considered as a therapeutic target for treatment of cancer.